The present application relates to an active material including a lithium composite oxide, to an electrode and a secondary battery that use the active material, and to a battery pack, an electric vehicle, an electric power storage system, an electric power tool, and an electronic apparatus that use the secondary battery.
In recent years, various electronic apparatuses such as a mobile phone and a personal digital assistant (PDA) have been widely used, and it has been demanded to further reduce the size and the weight of the electronic apparatuses and to achieve their long life. Accordingly, as an electric power source for the electronic apparatuses, a battery, in particular, a small and light-weight secondary battery capable of providing high energy density has been developed.
In these days, it has been considered to apply such a secondary battery to various other applications in addition to the foregoing electronic apparatuses. Examples of such other applications may include a battery pack attachably and detachably mounted on the electronic apparatuses or the like, an electric vehicle such as an electric automobile, an electric power storage system such as a home electric power server, and an electric power tool such as an electric drill.
Secondary batteries utilizing various charge-discharge principles to obtain a battery capacity have been proposed. In particular, a secondary battery obtaining a battery capacity with the use of insertion and extraction of an electrode reactant has attracted attention, since such a secondary battery provides higher energy density than lead batteries, nickel-cadmium batteries, and the like.
The secondary battery includes electrodes (a cathode and an anode) and an electrolytic solution. The cathode contains an active material (cathode active material) that inserts and extracts an electrode reactant. The anode contains an active material (anode active material) that inserts and extracts the electrode reactant. As the anode active material, a carbon material such as graphite has been widely used.
In order to apply the secondary battery to a large-size application such as the foregoing electric vehicle, it is necessary to increase size of a secondary battery as an electric power source, that is, it is necessary to use a secondary battery capable of producing high energy. Accordingly, as performance necessary for the secondary battery, safety capable of suppressing occurrence of defects such as ignitions is important in addition to capacity characteristics capable of obtaining a high battery capacity.
Therefore, in order to improve safety and the like, various studies have been heretofore made on configurations of secondary batteries. Specifically, it has been proposed to use a lithium composite oxide containing titanium (Ti) as a main component, more specifically, to use lithium titanate (Li4Ti5O12) as an anode active material (for example, see “Newly developed SCiB high-safety rechargeable battery,” Shinichiro Kosugi et al., Toshiba Review, Vol. 63, No. 2, pp. 54-57, 2008; and “New SCiB high-safety rechargeable battery for HEV application,” Norio Takami et al., Toshiba Review, Vol. 63, No. 12, pp. 54-57, 2008).
Charge-discharge electric potential of such Li4Ti5O12, that is, electric potential at which insertion and extraction of lithium (Li) as an electrode reactant occur is about 1.5 V based on electric potential at which precipitation and dissolution of lithium occur as a standard. Therefore, the charge-discharge electric potential of Li4Ti5O12 is significantly higher than general charge-discharge electric potential (from about 0.1 V to about 0.2 V both inclusive) of a carbon material such as graphite. In this case, in the case where Li4Ti5O12 is used as an anode active material, lithium metal is less likely to be precipitated on the surface of an anode at the time of charge. Thereby, internal short-circuit resulting from precipitation of the lithium metal is less likely to be generated, and accordingly, defects such as ignitions are less likely to occur. As a result, high safety is obtained thereby.
However, the charge-discharge electric potential of Li4Ti5O12 is excessively high. Therefore, while safety is improved, energy density as one of important factors determining a battery capacity is largely decreased. Therefore, in order to decrease the charge-discharge electric potential of Li4Ti5O12 in the range capable of securing safety, it has been proposed to substitute chromium (Cr) for part of Li4Ti5O12 (for example, see “Electrochemistry and Structural Chemistry of Li[CrTi]O4 (Fd3m) in Nonaqueous Lithium Cells,” Tsutomu Ohzuku et al., Journal of the Electrochemical Society, 147 (10), pp. 3592-3597, 2000; and “Comparative study of Li [CrTi]O4, Li [Li1/3Ti5/3]O4, and Li1/2Fe1/2 [Li1/2Fe1/2Ti]O4 in non-aqueous lithium cells,” Kazuhiko Mukaia et al., Journal of Power Sources, 146, pp. 213-216, 2005). Thereby, the charge-discharge electric potential is decreased by about 0.05 V.
As a lithium composite oxide, in addition to the compound containing Ti as a main component, a compound containing other transition metal such as manganese (Mn) as a main component has been proposed (for example, see Japanese Unexamined Patent Application Publication Nos. H10-172568, H11-071115, 2002-110225, 2012-099287, and 2012-059457, and Japanese Patent No. 4503160).